Cristiano Ronaldo scored for the eighth Serie A game in a row for Juventus on Sunday but could not prevent his side slipping to a 2-1 defeat to Napoli at the Stadio San Paolo.
The visitors saw more of the ball in the first half but did not manage a single effort on target. Their best chance was a wayward Ronaldo header just before half-time from a Paulo Dybala free-kick.
Napoli had the better chances, but both Arkadiusz Milik and Mario Rui sent efforts over the crossbar in an uneventful opening 45 minutes.
The hosts went ahead through Piotr Zielinski after 63 minutes. Goalkeeper Wojciech Szczesny could only parry a shot by Insigne, allowing the midfielder to tap home the opener.
Insigne volleyed home Napoli's second with four minutes of normal time remaining, and although Ronaldo pulled one back, it was to prove only a consolation.
Maurizio Sarri started with Blaise Matuidi in midfield, while Ronaldo, Gonzalo Higuain and Dybala all featured in attack. Gennaro Gattuso brought in goalkeeper Alex Meret for the injured David Ospina and also handed Fabian Ruiz a start:
Despite all the attacking talent on show, Juventus did not test goalkeeper Meret in the first half, as Napoli managed to keep the defending champions at bay.
Ronaldo did just get sight of goal in the first half but could not keep his header from Dybala's ball into the box down and saw it fly harmlessly over the bar.
The second half brought far more entertainment, although Juventus suffered an early blow when Miralem Pjanic was forced off through injury and replaced by Adrien Rabiot.
Sarri's men then thought they had taken the lead through Ronaldo. Dybala fired a shot off the post from Higuain's pass, and the Portuguese tapped the rebound into an empty net. However, his effort was ruled out for offside by Higuain in the build-up after a VAR check.
The chances then started to flow at both ends of the pitch. Zielinski curled an effort narrowly over the bar, while Higuain went for goal but saw his effort saved by Meret.
The breakthrough eventually came just after the hour mark. Insigne tried his luck from range with an effort that a diving Szczesny could only parry, allowing a grateful Zielinski to knock the rebound home (UK video only):
Sarri made a double change to try and get his team back in the game, sending on Douglas Costa and Federico Bernardeschi for Matuidi and Dybala.
Yet it was Napoli who scored again late on to double their lead. Insigne volleyed home Jose Callejon's cross to spark wild celebrations.
There was still time left for Ronaldo to maintain his fine goalscoring form. The 34-year-old stabbed in from close range to give Juve hope of a late comeback:
Sarri's men almost came up with a dramatic finish too. Higuain managed to get on the end of Juan Cuadrado's cross but put his overhead kick straight at Meret.
The result means it's a miserable return to Napoli for former boss Sarri. His side miss the chance to extend their lead at the top of the table to six points, while Napoli secure their first Serie A win of 2020 and move into 10th.
Juventus return to action at home to Fiorentina in Serie A on Sunday, February 2. Napoli play a day later at Sampdoria.